Additional technical limitations of reflectors

A large, bright rear-facing reflector can fail under a few conditions
in which a bright taillight would perform well.

Reflector performance suffers in fog, smoke, dust, heavy rain or snow, or
if the reflector is dirty or scratched.

The light a driver sees from a reflector has to make a round trip -- from
the car to the reflector and back. If the light path is partly obstructed, the
reflector's brightness is reduced more than a taillight's -- sometimes much more.

The effectiveness of a reflector depends on the direction of the light
beam, so anything that spreads the light beam -- beads of water or scratches on the
reflector's surface, fog, mist rain or smoke in the air -- can drastically reduce a
reflector's performance too.

Reflectors dim out as a large truck or bus gets closer.

Reflectors send back a narrow cone of light. In a large truck or bus, the
driver's eyes are several feet above the headlights. The driver's eyes
can be outside the cone of reflected light too soon, before the bicyclist begins to show
up in the headlight beams. The problem is worse with the newer, smaller U.S. Consumer
Product Safety Commission "long distance" reflectors.

Reflectors work poorly if a car's left headlight is out.

The angle between the right headlight and the driver's line of vision is
too large -- same problem as with the large truck or bus.

Reflectors work poorly if a car's headlights are dirty or poorly aimed.

Dirty or poorly aimed headlights reduce the amount of light reaching the
bicyclist's reflectors. Aim is a problem even with properly adjusted, clean headlights if
the road curves to the left, or if the car is descending a slope and the bicycle is
ascending the next one.

A hillcrest, car trunk or other obstruction can hide a reflector even
though it is in a driver's field of view.

The motorist must have a clear sight line to a reflector, but also the
car's headlight beams must reach it.

Though a low reflector does worsen the hilltop problem, I still
recommend mounting reflectors low so that low-beam headlights reveal them sooner under
most conditions. I recommend mounting a taillight high so it is visible
over obstructions. If you use both a reflector and a taillight, you can follow both of
these recommendations. You might also use additional, high-mounted reflective material,
for example on a helmet.